Method and compound for the selective adsorption of nitrogen oxides

ABSTRACT

A method for selectively absorbing nitrogen oxides NO x  from gaseaous mixtures containing carbon dioxide or carbon dioxide and water and optionally contaminants chosen among CO, SO 2 , hydrocarbons and mixtures thereof, comprising placing the gaseous mixtures in contact with absorber compounds having formula Ba 2 Cu 3 O 5+d , where d is a number from 0.6 to 1. New compounds having the above formula are characterized by high resistance to carbonatation and by specific Raman spectra.

TECHNICAL FIELD

The present invention relates to a compound and a method for the selective absorption of NO, nitrogen oxides from gaseous mixtures containing carbon dioxide.

In particular, it relates to the absorption of nitrogen oxides from the exhaust gas of internal-combustion engines.

BACKGROUND ART

The literature (M. Machida et al.—J. Chem. Soc., Chem. Commun. (1990), p. 1165, and New Frontiers in Catalysis, Proc. of the 10th Intern. Congress on Catalysis, Budapest, Hungary, Elsevier (1993) p. 2644) describes mixed barium-copper oxides which are given the formula BaCuO_(x), where x has the values of 2.1 and 25, and are capable of reversibly absorbing nitrogen oxides by working within a certain temperature range, fixing them as barium nitrites and nitrates, and of releasing them by heating to temperatures higher than the absorption values, restoring the structure of the initial oxides.

The above mentioned mixed oxides are highly reactive also to carbon dioxide, which they fix as highly stable barium carbonate which, by depositing on the surface of the material, inhibits its further absorbing capability.

High reactivity to carbon dioxide therefore prevents use of compounds BaCuO_(x) to absorb nitrogen oxides from mixtures rich in carbon dioxide, such as the exhaust gas of motor vehicles.

An attempt has been made to obviate this drawback by using mixtures of BaCuO_(2.1)/MnO₂ which are scarcely sensitive to carbonatation.

Finally, it has been found that BaCuO_(x) compounds tend to lose, over time, their capability of absorbing nitrogen oxides.

Application EP-A-666 102 describes the use of substances for adsorbing nitrogen oxides from the exhaust gas of engines designed to work with an excess of oxygen in the air/gasoline mix, capable of adsorbing NO and of converting it into NO₂ by virtue of the action of the excess oxygen that is present in the mix.

When the engine runs with an oxygen deficit (air/gasoline mix rich in gasoline), the adsorbed nitrogen dioxide reacts with the reducing gases that are present in the mix (CO and unburnt hydrocarbons), becoming N₂ and oxidizing the reducing gases to CO₂ and H₂O.

The adsorbers used in the European application are essentially constituted by mixtures of barium carbonate and copper oxide formed locally during preparation by decomposition of copper nitrate and barium acetate with Ba/Cu ratios within broad ranges (from 1:3 to 3:1).

Said adsorbers, however, are entirely inactive in fixing nitrogen oxides in the absence of oxygen or in case of oxygen deficit, such as when the engine, at startup, runs with gasoline-rich air/gasoline mixes.

Furthermore, the temperature window in which the adsorbers are active is shifted toward high temperatures, thus preventing adsorption when the engine is running cold.

WO 97/28884 discloses a compound of formula Ba₂Cu₃O₆ suitable for adsorbing gases, among others, carbon dioxide.

U.S. Pat. No. 5,238,913 reports that compounds of formula Ba₂Cu₃O_(5+x) (OL X L1) are suitable for preparing superconducting microcircuits. No indications are given about the method of preparation of the compounds and, in particular no mention is made of the compound Ba₂Cu₃O₆.

DISCLOSURE OF THE INVENTION

It has now been unexpectedly found that the compound having the formula Ba₂Cu₃O₆ and the Raman spectrum characteristics as set forth in the claims is capable of selectively absorbing nitrogen oxides NOx from gaseous mixtures rich in carbon dioxide, possibly containing pollutants such as CO, SO₂, hydrocarbons and mixtures thereof. Absorption occurs at temperatures between approximately 180° C. and 480° C., working at atmospheric pressure.

It has furthermore been found, and it is another aspect of the invention, that nitrogen oxide absorption kinetics is accelerated considerably by the presence of water vapor in the mixtures. In the case of NO₂, the presence of oxygen and moisture shifts the absorption toward relatively low temperatures comprised between approximately 180° C. and ambient temperature. Preferably, NO₂ absorption is performed at temperatures above 35° C.-40° C.

By effect of the absorption of considerable amounts of NO_(x) oxides, the compound of the invention decomposes forming barium nitrite and mono- and divalent copper oxides, if they are exposed to NO in the absence of oxygen, barium nitrate and bivalent copper oxide, if they are exposed to NO₂ or NO in the presence of oxygen.

The thermogravimetric curves plotted in FIGS. 1 and 2 show the absorption of NO and NO2 as a function of the temperature (absorption of mixtures of 25% NO and 3% O₂ in helium, with a space velocity of 3000/h and 2.5% NO₂ and 2% O₂ in helium/nitrogen with a space velocity of 3000/h and a heating rate of 20° C./min (percentages expressed by volume)).

By heating to temperatures above approximately 480° C., the compounds that have formed begin to decompose, releasing the nitrogen oxides and restoring the Ba₂Cu₃O₆ structure of the starting compound.

At temperatures above 480° C., barium nitrite and nitrate and copper oxide begin to react with each other, forming the compound Ba₂Cu₃O₆ and releasing, respectively, NO and NO₂ and possibly oxygen. In the range between 480° and 700° C., Ba₂Cu₃O₆ coexists alongside with barium nitrite and nitrate and with copper oxide; the Ba₂Cu₃O₆ fraction increases with time and temperature.

The selectivity of the Ba₂Cu₃O₆ with respect to CO₂ depends considerably on the preparation method.

It has been found, and it is another aspect of the invention, that the compound of the invention considerably increases its resistance to carbonatation if it is prepared starting from barium nitrate and copper oxide intimately mixed in a cationic ratio of 2:3, subsequently heating the mixture to 640° C.-650° C. in an air stream until the barium nitrate is completely decomposed and then cooling the mixture in air stream at a rate of no more than 20° C./min.

The air can be replaced with oxygen/nitrogen mixtures or oxygen/inert gas mixtures containing up to 25 g/m³ of water vapor and up to 400 ppm of CO₂.

It has furthermore been found that the presence of nitrogen oxides during the cooling of the material, or their addition to the reaction atmosphere to complete the synthesis, facilitate the formation of the carbonatation-resistant materials.

The curve of carbonatation as a function of temperature which is typical of the compound Ba₂Cu₃O₆ thus prepared as above specified is reported in FIG. 3 (stream of 10% CO₂, 10% H₂O, complement with mixtures of nitrogen and argon, exposure 5 hours, percentages by volume).

For comparison, the circles indicate the carbonatation behaviour of a non-resistant compound BaCuO_(2.5) prepared according to the methods described in literature.

The carbonatation curve of the compound supported on alumina is similar to the curve of the above mentioned compound. The preparation is made by immersing porous aluminum oxide, dehydrated beforehand, in a near-saturated solution of barium nitrate and copper nitrate in deionized water, using a barium ion/copper ion ratio of 2:3 and working at temperatures between 20° C. and 80° C.

The material, impregnated with the solution, is dried at 110° C.-150° C. and then subjected to the above described heat treatment (reaction at 640° C.-650° C. and then cooling at a rate of no more than 20° C./min).

The procedure can be repeated in order to increase the filling of the pores of the aluminum oxide until saturation is reached.

Approximately 3.5% by weight of supported compound is obtained for each impregnation/heat treatment cycle.

The curve of FIG. 3 shows that the compound Ba₂Cu₃O₆ prepared as mentioned above is not sensitive to carbonatation up to approximately 420° C. (less than 0.4% increase in weight after 5 h of exposure). The increase is less than 2% at 500 ° C., again after 5 h of exposure.

Resistance to carbonatation decreases considerably if the compound Ba₂Cu₃O₆ is prepared at 800° C. and then cooled quickly to ambient temperature (rate of approximately 5° C./sec).

Table 1 reports the weight increases by isothermal treatments in NO 1% by volume, 99% N₂ of Ba₂Cu₃O₆, in comparison with the “compound” Ba₂CuO_(2.5) prepared according to the methods described in literature.

TABLE 1 “BaCuO_(2.5)” 300° C. 400° C. 500° C. 12 hours 17.4% 17.1%  7.9% 36 hours 17.3% 17.4% 11.5% 60 hours 17.5% 17.4% 12.9% Ba₂Cu₃O₆ 300° C. 400° C. 500° C. 12 hours 17.8% 16.5% 11.7% 36 hours 19.5% 19.8% 19.3% 60 hours 21.7% 21.5% 18.2%

The table shows that the compound BaCuO_(2.5) ceases to absorb after approximately 12 h at temperatures between 300° C. and 400° C., whilst absorption continues at 500° C. Absorption at 500° C. is slightly more than half the absorption of Ba₂Cu₃O₆, which instead continues to absorb prolonged periods at all temperatures from 300° C. to 500° C.

The Raman spectrum of the carbonatation-resistant compound Ba₂Cu₃O₆ (prepared as herein before indicated) shown in FIG. 4 shows that the maximum intensity peak in the wave number range from 0 to 800 cm⁻¹ appears at wave number of 598±5 cm⁻¹ , and that at wave number 633±3 cm⁻¹ there is a mode whose intensity is between 0% and 30% of the intensity of the mode that appears at 598±5 cm⁻¹, or that said mode is absent.

It is also found that at wave number 560±5 cm⁻¹ there is a mode whose intensity is 30% less than the intensity of the mode that appears at 598±5 cm⁻¹. A symmetric band is centered around wave number 520±7 cm⁻¹ and has an intensity between 20% and 40% of the intensity of the mode that appears at 598±5 cm⁻¹.

The Raman spectra were recorded with a Dilor LabRam apparatus, using a laser beam at 632.8 nm with an intensity of 1 mW, focused on sample portions measuring 1 micron in diameter.

X-ray diffraction measurements of powders and of single crystals show that the compound Ba₂Cu₃O₆ crystalizes in the rhombic system, with cells characterized by the lattice parameters 4.18 ∈<a<4.35 Å, 6.83 ∈<a<7.33 Åand c=11.39±0.02 Å, which are the result of the distortion of a hexagonal packing in which 4.05 Å<a<4.28 Å, c =11.39±0.02 Å and the angle δ changes from 120° to a value between 115° and 118°.

The X-ray diffraction spectrum (powder diffraction) of the carbonatation-resistant compounds shows that the intensity of the reflections that can be detected at the angles 2 θ=29.7°×0.05° and 2θ=30.3°±0.05° is very weak and lower than 10% of the intensity of the intense reflection at 2θ=29.00°±0.05°. The lower the intensity of these reflections, the higher the resistance to carbonatation.

The powder X-ray diffraction measurements were made using a Philips X-pert diffractometer constituted by a PW1830/40 generator, PW3719 goniometer and PW3710 control unit using Cu Kα radiation.

Advantageously, in order to increase the exposed surface area, the compounds used in the absorption method of the invention are supported on porous carriers having surface area higher than 50 m²/g preferably higher than 100 m²/g and more preferably comprised in the range of 150-500 m²/g, which are inert towards the reactants used for preparing the compounds.

Examples of said carriers are alumina, titania, zirconia, boron nitride, silicon carbide.

As mentioned, the compounds according to the invention are applied particularly in the absorption of NO_(x) oxides from the exhaust gas of internal-combustion engines.

By virtue of the capability to absorb and desorb oxides at temperatures in the range between approximately 200° C. and 700° C., the compounds are used in mufflers preferably placed in a portion of the exhaust pipe which is at a temperature between approximately 200° C. and 500° C. when the motor is running cold and at temperatures above approximately 550° C. when the motor is running steady.

Another application of interest of the compounds relates to the absorption of nitrogen dioxide (NO₂) from the fumes of plants such as those for nitric acid and for preparing silicon.

Other applications of the compounds relate to the absorption of NO_(x) oxides from the exhaust fumes of domestic heating systems or from fuel-burning electric power stations.

In the case of the absorption of nitrogen oxides from the exhaust fumes of fixed plants, such as heating systems or fuel-burning power stations, the compounds Ba₂Cu₃O5+d, once they have been converted into Ba nitrites and nitrates, can be restored to the initial fully active form by heating.

It has been found that the compounds Ba₂Cu₃O_(5+d) which have already been subjected to absorption of NO_(x) oxides and have not been fully decomposed into barium nitrate and cupric oxide oxidize hydrocarbons to CO₂ and H₂O and CO to CO₂ even in the absence of oxygen at temperatures lower than those of pure compounds Ba₂Cu₃O_(5+d).

Following test illustrates the above behaviour.

Since the compounds decompose into barium nitrate and cupric oxide which do not contribute to the catalytic reaction, the maximum activity is found in the materials which have been exposed to NO_(x) oxides just until the decomposition starting point.

Porous alumina was impregnated in a solution of Ba(NO₃)₂ and Cu(NO₃)₂ in a cationic ratio of 2:3, dried at 150° C. and then treated at 650° C. until full nitrate decomposition was achieved. The resulting product was quickly cooled to ambient temperature and was found to be constituted by 3.5% by weight of the compound Ba₂Cu₃O_(5+d). Part of this material was exposed for 1 hour to a stream of gas composed of 90% synthetic air, 2% H₂O and 8% NO₂. Under these conditions, approximately 50% of the compound Ba₂Cu₃O₆ decomposed to barium nitrate and copper nitrate. The material was then heated in air at 250° C. to convert the copper nitrate into copper oxide. The treatment with oxygen can be omitted when the material is used in the oxidation reaction at 250° C. or higher temperatures.

0.5 grams of material thus prepared were introduced in a reactor to measure the catalytic yield in methane oxidation. The gas in the reactor was constituted by a mixture of methane/oxygen/nitrogen in the proportions 2/18/80, flowing at 700 cc/min (84000/hour).

As the temperature increased, the yields listed in Table 2 were found; these yields are expressed as the percentage of methane converted into CO₂+H₂O and compared with the yields of a sample of the same material in pure form.

It has been found that the activity of the compounds Ba₂Cu₃O₅+d can be significantly increased by promoting them with oxides selected from cerium oxide, zirconium oxide and the oxides of the rare earth metals particularly lanthanium and cerium. The amount of the promoter generally is up to 10% by weight expressed as metal.

TABLE 2 Yield of the material Yield of material Temperature after NO₂ absorption in pure form (° C.) (%) (%) 300 0.0 0.0 350 3.0 0.0 400 3.8 1.6 450 9.8 6.7 500 20.0 16.1 550 91.8 40.2 600 100.0 100.0 

What is claimed is:
 1. A compound Ba₂Cu₃O₆ characterized by a Raman spectrum having a maximum intensity peak in the wave number range from 0 to 800 cm⁻⁴ of a mode at a ware number of 598±5 cm⁻⁴, and in which at wave number 633±3 cm⁻⁴ here is a mode whose intensity is at least 30% lower than the intensity of the mode at 598±5 cm⁻⁴, or no mode is present.
 2. The compound according to claim 1, wherein in the Raman spectrum there is, at wave number 560±5 cm⁻¹, a mode whose intensity is 30% less than the intensity of the mode at 598±5 cm⁻¹.
 3. The compound Ba₂Cu₃O₆ having the Raman spectrum as characterized in claim 2, which comprises a symmetric band which is centered at wave number 520±7 cm⁻¹ and has an intensity between 20% and 40% of the intensity of the mode at 598±5 cm⁻¹.
 4. The compound according to claim 1, supported on inert porous carriers having surface area higher than 50 m²/g.
 5. The compound according to claim 1, which has been exposed to NO_(x), oxides until a point of not complete decomposition of the compound to barium nitrate and cupric oxide is reached.
 6. The compound according to claim 5, wherein the exposition to NO_(x) oxides has been discontinued at the point of the starting decomposition of the compound.
 7. The compound according to claim 1, containing a promoter selected from the group consisting of cerium oxide, zirconium oxide and the oxides of the rare earth metals.
 8. The compound Ba₂Cu₃O₆ according to claim 1, characterized by resistance to carbonatation in a stream of 10% CO₂, 10% H₂O, the complement being a mixture of nitrogen and argon, measured by the increase in weight of the compound, of less than 0.4% at 420° C. after 5 hours of exposure and less than 2% at 500° C. again after 5 hours of exposure.
 9. A process for the preparation of the compound of claim 1, comprising heating a mixture of barium nitrate and copper oxide in a cationic g-atom ratio of 2:3 at a temperature of 640°-650° C. in air stream until barium nitrate is completely decomposed and then cooling the reaction mixture in air stream at a rate of no more than 20° C./min.
 10. The process according to claim 9, wherein the air stream is replaced with oxygen/nitrogen mixtures containing up to 25g/m³ of water vapor and up to 400 ppm of CO₂.
 11. The process according to claim 9, wherein the reaction atmosphere and/or the cooling atmosphere is added with nitrogen oxides.
 12. A compound Ba2Cu3O6 obtainable by a method according to claim 9, characterized by a Raman spectrum wherein a mode at a wave number of 598+/−5 cm⁻¹ has a maximum intensity in the wave number range from 1 to 800 cm⁻¹, and a mode at a wave number 633+/−3 cm⁻¹ is absent or has an intensity at least 30% lower than the intensity of the mode at 598+/−5 cm⁻¹.
 13. A compound Ba2Cu3O6 characterized by a Raman spectrum wherein a mode at a wave number of 598+/−5 cm⁻¹ has a maximum intensity in the wave number range from 1 to 800 cm⁻¹, a mode at a wave number 633+/−3 cm⁻¹ is absent or has an intensity at least 30% lower than the intensity of the mode at 598+/−5 cm⁻¹, and a mode at a wave number 560+/−5 cm⁻¹ has an intensity at least 30% lower than the intensity of the mode at 598+/−5 cm⁻¹, and which comprises a symmetric band which is centered at wave number 520+/−7 cm⁻¹ and has an intensity between 20% and 40% of the intensity of the mode at 598+/−5 cm⁻¹, the product being obtainable by a method according to claim
 9. 14. A method for selectively absorbing nitrogen oxides NOx from gaseous mixtures containing carbon dioxide or carbon dioxide and water, comprising contacting the gaseous mixtures with absorbers comprising a compound Ba₂Cu₃O₆ having the characteristics set forth in claim
 1. 15. The method according to claim 14, wherein absorption is performed in the presence of oxygen.
 16. The method according to claim 14, wherein absorption is performed at temperatures between 180 and 480° C.
 17. The method according to claim 14, wherein the nitrogen oxides are absorbed from gas mixtures formed of the exhaust gas of internal-combustion engines.
 18. The method according to claim 17, wherein the compounds are used in mufflers located in a portion of the exhaust pipe that is at temperatures between 200° C. and 550° C. when the engine runs cold and in a portion that reaches temperatures above 550° C. when the engine runs warm.
 19. A method for absorbing NO₂ from exhaust fumes of nitric acid plants or silicon production plants, wherein the fumes are passed over absorbers which comprise a compound according to claim
 1. 20. The method according to claim 19, wherein absorption is performed at a temperature between 40 and 180° C.
 21. A method for absorbing NO_(x) nitrogen oxides from the exhaust fumes of domestic heating systems or fuel-burning power stations, wherein the fumes are passed over absorbers which comprise a compound according to claim
 1. 